Yoma 11A

כל השערים שהיו שם לא היה להם מזוזה, חוץ משער ניקנור, שלפנים ממנו לשכת פרהדרין.

All the gates that were there on the east side of the Temple courtyard did not have a mezuza except for the Gate of Nicanor, as in the courtyard just inside the gate was the Chamber of Parhedrin, in which there is an obligation to affix a mezuza. Therefore, a mezuza was affixed to the gate as well.

לימא רבנן היא ולא רבי יהודה? דאי רבי יהודה, היא גופה גזירה, ואנן ניקום ונגזור גזירה לגזירה? אפילו תימא רבי יהודה, כולה חדא גזירה היא.

Let us say that the baraita is in accordance with the opinion of the Rabbis and not in accordance with the opinion of R' Yehuda, as, if it were in accordance with the opinion of R' Yehuda, a difficulty arises. The principle is that decrees are issued only to prevent violation of a Torah prohibition. The fact that a mezuza was affixed to the Parhedrin chamber itself is due to a rabbinic decree, and will we then proceed to issue a decree to affix a mezuza on the gate before the chamber in order to prevent violation of the existing decree? The Talmud rejects this reasoning: Even if you say that the baraita is in accordance with the opinion of R' Yehuda, it is not difficult, as the entire obligation to affix the mezuza on both the chamber and the gate is the result of a single decree.

תנו רבנן: ״בשעריך״⁠ — אחד שערי בתים ואחד שערי חצירות ואחד שערי מדינות ואחד שערי עיירות יש בהן חובת מצוה למקום, משום שנאמר: ״וכתבתם על מזוזות ביתך ובשעריך״.

§ Apropos the mezuza in the High Priest’s chamber, the Talmud discusses other halakhot of mezuza. A baraita states with regard to the verse: “And you will write them upon the doorposts of your houses and upon your gates” (Deuteronomy 6:9): With regard to the gates of houses, and the gates of courtyards, and the gates of cities, and the gates of towns, all of them are obligated in the mitzva of mezuza in that place, due to the fact that it is stated: “And you will write them upon the doorposts of your houses and upon your gates.”